Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/10076—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating using optical phase conjugation, e.g. phase conjugate reflection

Definitions

• the present invention relates generally to lasers employing phase conjugate reflecting elements, and in particular, to phase conjugate lasers wherein the phase conjugate reflector is pumped by means of pulsed laser energy.
• phase conjugation involves the use of optically nonlinear media, which may be activated by means of incident laser energy to restore severely distorted optical beams to their original unaberrated state.
• Concetto R. Giuliano entitled “Applications of Optical Phase Conjugation", Physics Today, April 1981.
• phase conjugation has also been employed to form a laser resonator.
• a laser resonator Such a device is disclosed in U.S. Patent No. 4,233,571 entitled "Laser Having a Nonlinear Phase Conjugating Reflector".
• the laser disclosed herein is one in which one of the con ⁇ ventional end mirrors of the laser resonator has been replaced by a phase conjugate reflector.
• phase conjugate reflectors may be employed, which utilize four-wave mixing, three-wave mixing, simulated Raman scattering, stimulated Brillouin scattering, or photon echo processes.
• phase conjugate reflectors in optical resonators to produce a new class of lasers whose output is charac ⁇ terized by beams of high spatial quality.
• phase conjugate reflectors as well as intracavity amplifiers, maintain moderately high values of reflection and amplification, respectively, over time durations that are at least an order of magnitude larger than the resonator cavity round-trip time.
• This constraint often imposes severe optical energy requirements on the lasers used to pump the phase conjugate reflectors and amplifiers.
• the high peak powers needed over long time durations often result in thermal problems, making CW operation with high speed phase-conjugate reflectors very difficult.
• the present invention incorporates a lasing medium, phase-conjugate reflector and an output coupling device appropriately disposed along an optical path to form a laser resonator.
• a laser resonator having a conventional end mirror and a phase-conjugate reflector as the other end mirror may be considered a phase- conjugate laser, for the purposes of the present invention.
• the output coupling device forms one end mirror of the laser while the phase-conjugate reflector forms the other end mirror.
• the output coupler is such that it reflects a substantial portion of the energy incident thereupon and transmits a portion of the laser energy to form an output beam of the laser.
• the phase-conjugate reflector is utilized as the second end mirror of the laser resonator and it reflects the phase conjugate of energy incident thereupon. This portion of the present invention is substantially the same as the device disclosed in U.S. Patent No.
• the improvements provided by the present invention include the use of a pulsed pump source which applies laser pulses to the phase-conjugate reflector.
• the laser pulses have relatively high peak power and rela ⁇ tively low average power.
• the pulses have a predeter ⁇ mined periodicity which matches the round- trip time for laser energy traversing the laser resonator.
• the pump pulses are applied to the phase-conjugate reflector in synchronism with the energy tranversing the resonator.
• FIG. 1 illustrates a first embodiment of a synchro ⁇ nously-pumped phase-conjugate laser in accordance with the principles of the present invention
• FIG. 2 shows a more detailed illustration of the laser of FIG. 1, particularly employing four-wave mixing as the phase conjugation process
• FIG. 3 illustrates a self-pumped embodiment of a laser in accordance with the principles of the present invention.
• the laser 20 comprises a phase-conj ugate reflector 21 , lasing medium 22 , and output coupling device 23 disposed along an optical path.
• a laser resonator is formed be tween the phase- conjugate re flector 21 and the output coupling device
• the phase-conj ugate re flector 21 may be any of a number of phase-conj ugate re flecting devices which employ four-wave mixing , three-wave mixing , or photon echo processes , or the like . These processes are well-known in the art and are described in a variety of patents and publications including "Applications of Optical Phase Conj ugation" and U . S . Patent No . 4 , 233 , 571 , c ited above .
• the output coupling device 23 may be a partially reflecting and partially transmitting mirror as is present in a conventional laser resonator, or any other conventional device known in the art.
• the lasing medium 22 may be any conventional lasing medium such as a dye, neodymium YAG (Nd:YAG), CO2, or ruby, or the like. It is to be understood that the phase-conjugate reflector 21, lasing medium 22 and output coupling device 23 are all selected to have appropriate operating frequency regimes which are compatible.
• the Giuliano publication includes a table which illustrates a variety of lasing media, phase conjugation processes, nonlinear media, and measured reflectivities of the phase-conjugate reflectors for a variety of laser wavelengths.
• a pulsed pump source 24 and a pump source 25 are provided to pump both the phase-conjugate reflector 21 and lasing medium 22, respectively.
• pump source 25 may, in addition, be pulsed in a manner similar to the pulsed pump source 24.
• the alternative nature of pump source 25 will be discussed in more detail below.
• the pulsed pump source 24 may be a conventional mode-locked laser which is compatible with the nonlinear medium employed in the phase-conjugate reflector 21.
• the pulsed pump source 24 may be a Q-switched mode-locked neodymium YAG laser used to generate four-wave mixing in the phase conjugate reflector 21.
• the nonlinear medium employed in the phase-conjugate reflector 21 may be silicon, or a saturable absorber, or the like.
• pump source 24 for example, pump source 24
• this source drives the system, and hence controls the timing of the resonator.
• the pump source 25 is also a pulsed source, it must have the appropriate timing requirements in relation to the pulsed pump source 24
• pulsed energy provided by the pulsed pump source 24 is such that it has a relatively high peak power and a relatively low average power.
• duration of the pulses provided by the pulsed pump source 24 vary depending upon the selection of components used in the laser 20.
• both pump sources 24, 25 may be provided by means
• the resonator may be configured such that the optical energy requirement necessary for pumping are held to a minimum.
• the master oscillator may include the use of an optical frequency shifting
• • ->" device if two separate wavelengths are desirable for use in the phase conjugate reflector 21 and lasing medium 22. If the two pump sources 24, 25 are operated in a pulsed mode and are not of common orgin (master oscillator) , a timing circuit arrangement or an optical 35 delay line may be employed to control the synchronization and relative timing of the two pump sources 24, 25 as is required for optimal operation.
• FIG. 2 shows a specific embodiment of the present invention which employs four-wave mixing as the phase conjugation process in the phase-conjugate reflector 21.
• One specific system may employ a dye as the lasing medium 22, which may be pumped in a continuous manner by an argon ion or doubled YAG laser, or pumped in a synchronously-pumped manner by a mode-locked laser used as the pump source 25.
• the nonlinear medium 26 in the phase-conjugate reflector 21 may be sodium gas which is pumped by the pulsed pump source 24, which may be a dye laser.
• a CW dye laser has been experimentally demonstrated incorporating a phase-conjugate mirror.
• This laser system did not employ pulsed pump sources as is provided by the present invention.
• the output coupling device 23 may be a mirror arrangement which reflects a portion, say 80% of the energy and transmits the remainder as an output beam 35 of the laser 20.
• the pulsed pump source 24 applies a series of short laser pulses having high amplitude and relatively low average power to the nonlinear medium 26.
• Each laser pulse is split by means of a beamsplitter 30, or the like, and applied to opposite ends of the non ⁇ linear medium 26 as pulses 31, 32. It is to be under ⁇ stood that the optical path length that each of the pulses 31, 32 traverse prior to impinging upon the nonlinear medium 26 should closely matched in order to ensure that the pulses applied to the two ends of the nonlinear medium 26 are timed to arrive at the medium 26 within a duration shorter than the coherence time thereof.
• Phase conjugation occurs in the nonlinear medium 26 at times when both pump pulses 31, 32 are incident upon the nonlinear medium 26 within a time duration less than the coherence time thereof.
• the coherence time of the nonlinear medium 26 corresponds to a relaxation time in which the medium 26 feels the effects of an incident pulse.
• a third pulse 33, or probe pulse 33 which represents the intracavity power in the laser 20, and which undergoes phase conjugation, must also be timed appropriately.
• the presence of all three pulses 31, 32, 33 in the nonlinear medium will generate a backscattered pulse 34 which is the phase conjugate of the probe pulse 33.
• the nonlinear medium 26 is active in producing phase conjugated energy when the three laser pulses 31, 32, 33 are present in the nonlinear medium 26 with relative time delays that are less than appropriate relaxation times in the nonlinear medium 26.
• the phase-conjugate reflector 21 is hence turned on and off based upon the duration and period of the applied laser pulses 31, 32, 33 and the relaxation times of the nonlinear medium 26.
• the overall gain of the laser system is modulated so as to have a relatively low duty cycle. This keeps the overall energy requirements of the system low, while the high
• OMP phase-conjugation efficiencies, and high peak powers of the pump laser pulses allow for high peak powers to be provided as the output pulse 35 of the laser 20.
• the response time of the phase-con ugate reflector 21 and the duration of the laser pulses 31, 32, 33 in the nonlinear medium 26 should be much smaller than the cavity round-trip time.
• the cavity round-trip time is matched to the period of the pulse train provided by the pulsed pump source 24.
• pump source 25 is operated in a pulsed mode, its pulses must be applied to the lasing medium 22 at the proper time when the laser pulse 33 is traversing through the lasing medium 22.
• the pulses which circulate in the laser resonator represented by pulse 33 and reflected pulse 34 arrive- at the lasing medium 22 and nonlinear medium 26 at the exact times when the respective media 26, 22 are turned on by means of their respective pump sources 25, 24. For the remainder of the period, the phase-conjugate reflector 21 and lasing medium 22 are turned off.
• the reflectivity pro ⁇ vided by the phase-conj gate reflector 21 may be quite high and may also exceed unity. Such is the situation when using a pulsed dye laser as pulsed pump source 24 and sodium as the nonlinear medium 26.- Other examples may be found in the table of the Giuliano publication cited above. With reference to FIG. 3, when the reflectivity of the phase conjugate reflector 21 is large and the gain of the lasing medium 22 can be made high enough, the requirement for the pulsed pump source 24 may be eliminated once the system is in operation.
• the feedback loop may be provided by a beamsplitter, or the like, in order to couple a portion of the output pulse back to the phase-con ugate reflector 21.
• the output coupling device 23 must transmit a larger portion of the energy, which in turn is partially coupled back to the phase conjugate reflector 21.
• An external gain medium 37 may be employed in the feedback loop to ensure that the pump beams have the required energy. In this configuration, the pump source 25 may still be required to generate the inverted population in the lasing medium 22.

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• Physics & Mathematics (AREA)
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• Optics & Photonics (AREA)
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Citations (4)

• 1982
  • 1982-05-20 US US06/379,837 patent/US4493086A/en not_active Expired - Lifetime
• 1983
  • 1983-04-12 IL IL8368365A patent/IL68365A/xx not_active IP Right Cessation
  • 1983-04-14 WO PCT/US1983/000561 patent/WO1983004144A1/en active IP Right Grant
  • 1983-04-14 DE DE8383901562T patent/DE3368822D1/de not_active Expired
  • 1983-04-14 EP EP83901562A patent/EP0109411B1/de not_active Expired
  • 1983-05-18 IT IT48319/83A patent/IT1180653B/it active

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